Netcat is a computer networking service for reading from and writing
network connections using TCP or UDP. Netcat is designed to be a
dependable "back-end" device that can be used directly or easily driven
by other programs and scripts. At the same time, it is a feature-rich
network debugging and investigation tool, since it can produce almost
any kind of correlation you would need and has a number of built-in
capabilities

Although NetCat is not the state of the art tool anymore and it sends
all packets uncrypted through the net, it´s still a very good tool for
easy banner grabbing, binding shells or reverse shells.